Web pages are often displayed with advertisements relating to the web pages. For example, if a user is viewing a web page relating to a particular topic, the user may be interested in products and services also relating to that topic. An online advertising service can provide a way for advertisers to display advertisements to those users. Often, the online advertising service requests data from the advertisers about which advertisements are successful. This information may be used by the online advertising service to generate statistical information about the advertisements and their effectiveness.